Photoelectron spectroscopy of doubly charged anions: Intramolecular Coulomb repulsion and solvent stabilization

Photoelectron spectroscopy has been combined with an electrospray ionization technique to study gas-phase multiply charged anions and their intramolecular Coulomb repulsion and solvation stabilization. We report an initial study on a series of linear dicarboxylate dianions (DC2-), -O2C(CH2)(n)CO2- (n = 3-6), solvated by one and two waters. The second electron binding energy of DC2- decreases with decreasing aliphatic chain length owing to the increasing Coulomb repulsion between the two charges; the dianion with n = 2 has a binding energy close to zero and is not stable in the gas phase. We found that this dianion can be stabilized by one water, and in general the first water stabilizes the electron binding energy of DC2- by similar to 0.3 eV. The second water, however, was observed to show a much stronger stabilization effect (similar to 0.6 eV), suggesting that the two waters solvate the two carboxylate groups separately.